In a cellular mobile communication system, an evolution from a UMTS (universal mobile telecommunication system) to an LTE (long term evolution) has been devised. In the LTE, an OFDM (orthogonal frequency division multiplexing) and an SC-FDMA (single carrier-frequency division multiple access) are adopted respectively as downlink and uplink radio access technology, thereby enabling a high-speed radio packet communication to be performed at 100 Mb/s or higher for a downlink peak transmission rate and 50 Mb/s or higher for an uplink peak transmission rate. In the 3GPP (3rd Generation Partnership Project) as an international standardization organization, a study of a mobile communication system LTE-A (LTE-Advanced) based on the LTE has been started to realize a further high-speed communication. In the LTE-A, the downlink peak transmission rate of 1 Gb/s and the uplink peak transmission rate of 500 Mb/s are aimed at, and various new techniques are studied on a radio access system, a network architecture, etc. (3GPP TR 36. 913 V8. 0. 1 (2009-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced) (Release 8), 3GPP TR 36. 912 V9. 0. 0 (2009-09), 3rd Generation Partnership Project; Technical Specification group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9) and 3GPP TS 36. 133 V9. 2. 0 (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management (Release 9)). Note that, since the LTE-A is based on the LTE, it is devised to maintain backward compatibility.
As one of the methods for establishing a high-speed data communication, the method of deploying a relay station (relay node (RN)) as illustrated in FIG. 1 has been studied to support the communication between a base station and a mobile station (3GPP TR 36. 912 V9. 0. 0 (2009-09), 3rd Generation Partnership Project; Technical Specification group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9)). The relay station relays the communication between a base station (Doner eNB or eNB) and a mobile station (user equipment (UE)), and is provided to support a high-speed data communication. As illustrated in FIG. 2, the link between the mobile station UE and the relay station RN is referred to as a Uu, and the link between the base station (eNB) and the relay station (RN) is referred to as a Un. In the following explanation, the Uu may be referred to as an access link, and the Un may be referred to as a backhaul link.
Various schemes can be implemented to embody a relay station, but for example, a repeater scheme, a decode and forward scheme, an L2 scheme, and an L3 scheme have been studied. The relay station in the repeater scheme has only the function of amplifying a radio signal (data signal and noise). The relay station in the decode and forward scheme has the function of amplifying only a data signal in the radio signal. The relay station in the L2 scheme has the function of the L2 such as a MAC layer etc. The relay station in the L3 scheme has the function of the L3 such as an RRC layer etc., and functions like a base station. The relay station in the L3 scheme is referred to as a Type1 RN in the LTE-A.
A method of evolving a relay station in to a cell is also studied. For example, a method of evolving a relay station to be provided at a cell edge to increase the throughput of the cell edge, a method of evolving a relay station to be provided in a range where radio waves do not reach from the base station locally in a cell (dead spot), etc. are studied.
When data is transmitted between the base station and the mobile station through the relay station (Type1 RN) of the L3 scheme, it is preferable that no self-interference is generated in the relay station in inband relaying in which the same frequency band is shared between the base station and the relay station, and between the relay station and the mobile station. The self-interference (or also called “loop interference”) refers to interference occurring when the relay station receives DL data from the base station to the relay station and simultaneously transmits downlink data to the mobile station, and the transmission data appears in a receiver of the relay station, thereby generating interference with the data from the base station. Likewise with the uplink data, there can occur the self-interference. When the self-interference occurs, the relay station cannot correctly receive data.
To overcome the problem of the self-interference, the following policies are studied for LTE-A (3GPP TR 36. 912 V9. 0. 0 (2009-09), 3rd Generation Partnership Project; Technical Specification group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9).
(A) Downlink: The relay station does not transmit data to the mobile station in the DL backhaul as a subframe for receiving data from an upper base station.
(B) Uplink: The relay station does not receive data from the mobile station in the UL backhaul as a subframe for transmitting data to an upper base station.
Based on the policy (A) above, as illustrated in FIG. 3, when the downlink backhaul is set between the relay station and the base station, the subframe between the relay station and the mobile station is set as an MBSFN (multicast/broadcast over single frequency network) subframe because, in the MBSFN subframe, the mobile station for the LTE does not receive unicast data. Therefore, since the mobile station UE does not receive a part of a reference signal, it is preferable because it is not necessary to make an unnecessary measurement of the reference signal in the mobile station. That is to say, the relay station can transmit a PDCCH (physical downlink control channel), a PHICH (physical hybrid ARQ indicator channel), a PCFICH (physical control format indicator channel) while it cannot transmit a PDSCH. To receive the control signal, a reference signal is arranged in the first half (CTRL section illustrated in FIG. 3) of the MBSFN subframe, but it is not arranged in the last half of the MBSFN subframe.
Based on the policy (B) above, control is performed in the relay station not to grant the mobile station permission to transmit uplink data before 4 subframes (4 ms) in the UL backhaul because if the mobile station is granted the permission to transmit uplink data before 4 ms in the uplink backhaul, the mobile station transmits data to the relay station in the uplink backhaul, which is to be avoided.
Furthermore, in the relay station, control is performed not to transmit downlink data to the mobile station before 4 subframes (4 ms) in the uplink backhaul for the following reason. That is, in the HARQ (hybrid automatic repeat request) of the LTE, it is regulated that a receiving station is to return an ACK/NACK signal in 4 ms (4 subframes) after a transmitting station transmits data. Therefore, if downlink data is transmitted to the mobile station in 4 ms in the uplink backhaul, the mobile station transmits the ACK/NACK signal to the relay station in the uplink backhaul, which is to be avoided.
In the uplink backhaul, a PUCCH (physical uplink control channel) and a PUSCH (physical uplink shared channel) as control signals to the relay station can be transmitted, but the PUCCH and the PUSCH as control signals from the mobile station cannot be transmitted.
As illustrated in 3GPP TR 36. 912 V9. 0. 0 (2009-09), 3rd Generation Partnership Project; Technical Specification group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced) (Release 9), a backhaul is discussed with regard to the LTE-A. There is made a study on whether to establish downlink and uplink backhauls in which subframe of a radio Frame in the LTE-A. Suppose that a backhaul is always fixedly configured in a position of the same subframe in a radio Frame. When considering a relationship between a HARQ (Hybrid Automatic Repeat reQuest) and performance timing, there arise the following problems. These problems will be described in detail below. Suppose that in the following description, as illustrated in FIG. 4, a radio frame having a duration of 10 ms is composed of ten subframes #0 to #9 each having a duration of 1 ms as a TTI (Transmission Time Interval).
An example in the case where a backhaul is always configured in a position of the same subframe in a radio Frame will be described with reference to FIG. 5. In the continuing Frames (Frame_0, Frame_1, Frame_2, Frame_3, . . . ), FIG. 5 illustrates setting of 1 ms unit or timing of operations of (a) a downlink backhaul DL_BH, (b) a downstream access link DL_AL, (c) an uplink backhaul UL_BH, (d) an upstream access link UL_AL, and (e) HARQ processes (process numbers PID1, . . . , PID8) of access link. In FIG. 5, a downward arrow indicates transmission of a downlink signal, and an upward arrow indicates transmission of an uplink signal.
In (a) to (d) of FIG. 5, black-filled portions mean that backhauls or access links are incapable of being configured. In (a) of FIG. 5, for example, since downstream access links are used in the subframes #0, #4, #5, and #9 for transmission of control data, the downlink backhauls are incapable of being configured in these subframes. Therefore, in this example, downlink backhauls are configured in the subframes #1 in all of the continuing Frames. In the specifications of the LTE, since an ACK/NACK signal is sent back after 4 ms of data transmission, the relay station RN sends back the ACK/NACK signal to the subframe #5 with respect to the data transmission through the base station eNB in the subframe #1. As a result, in (c) of FIG. 5, the uplink backhauls are configured in the subframes #5. In (a) and (c) of FIG. 5, durations of the downlink or uplink backhauls are highlighted by solid thick frame lines.
When the downlink backhauls are configured in the subframes #1 and the uplink backhauls are configured in the subframes #5, access links are incapable of being configured in the same subframes. Therefore, as illustrated in (b) and (d) of FIG. 5, portions of the subframes #1 are displayed (incapable of being configured) to be black-filled in the downstream access links. On the other hand, portions of the subframes #5 are displayed to be black-filled (incapable of being configured) in the upstream access links.
There are two problems in the case where a backhaul is always configured in a position of the same subframe in one Frame as illustrated in FIG. 5.
First, a first problem is that backward compatibility with the LTE is lost. As described above, in the specifications of the LTE, an ACK/NACK signal is sent back after 4 ms of the data transmission. However, when the backhaul is configured as illustrated in FIG. 5, the ACK/NACK signal is to be sent back after 6 ms, and therefore the specifications of the LTE are not satisfied. In the example of FIG. 5, the ACK/NACK signal from the base station eNB toward the data transmission through the uplink backhaul (subframe #5) corresponds to the downlink backhaul (subframe #1) of the next Frame. However, when the backward compatibility with the LTE need not be maintained with regard to the reply timing of the HARQ, the above matter does not become a big problem.
Next, a second problem is as follows. That is, in the configuration of the backhaul illustrated in FIG. 5, a HARQ process in which the HARQ of an access link is incapable of being performed and duration of the HARQ process are scattered. Therefore, effective scheduling of the access link becomes difficult in the relay station RN.
In the example illustrated in FIG. 5, a part (four portions; illustrated by thick lines) of the HARQ processes of the process numbers PID2, PID4, PID6, and PID8 are incapable of being used. Specifically, in the HARQ processes of the process numbers PID2, PID4, PID6, and PID8, timing points of the uplink data transmission are matched with the uplink backhauls of the Frame_2, Frame_3, Frame_0, and Frame_1, respectively, and therefore the access links are incapable of being used. Accordingly, in the case of performing new data transmission, particular duration scattered as illustrated in FIG. 5 of the HARQ process in which the HARQ is incapable of being performed are avoided and scheduling is to be configured. As a result, there are problems that complexity of the scheduling is increased and the efficiency of the access link is reduced.